This invention relates to a motor starting switch and more particularly to a motor starting switch which is intended for use with a two speed electric motor.
In the operation of two speed electric motors, it is common for the motors to have three sets of windings incorporated in their stator assemblies: a high speed winding having, for example, four poles; a low speed winding having, for example, six poles; and a starting or auxiliary winding. In many motor applications it is desirable that the motor always be started in such a manner as to generate the greatest possible starting torque. Generally, the motor will generate the greatest starting torgue when it is started in its highest speed mode of operation (e.g., when the four pole windings of the above-described motor are energized). However, with certain prior art motor starting switches, timing of the movement of the contacts is required as the switch moves from its starting to its run position so as to prevent line current from being broken. The breaking of the line current to the motor oftentimes results in arcing across the switch contacts thus causing a decrease in the service or operating life of the motor starting switch with a consequent decrease in the service life of the motor. It is also difficult to ensure that the required timing of the motor starting switch can be maintained over the service life of the motor.
Also, in certain prior art two speed motor starting switches, jumpers were used within the switch so as to energize the starting or auxiliary winding when the starting switch was in its starting position. These jumpers added to the complexity and the expense of the starting switch.
In other prior art motor starting switches, excessive force could be applied to one or more of the movable switch arms as the switch was moved from its run to its starting position. This in turn caused excessive bending stresses to be applied to the switch arm which, on occasion, led to the premature failure of the switch arm upon repeated usage. Also, in certain prior art two speed motor starting switches a high force level was required for actuation of the switch. In certain instances, the force required to actuate the switch was nearly equal to the force the centrifugal actuator could apply to the starting switch. If certain of these prior art motor starting switches were not accurately positioned relative to the centrifugal actuator, the actuator would not properly open and close the switch and could cause failure of the motor in which the switch is installed.
Reference may be made to U.S. Pat. No. 3,381,197 which discloses a motor starting switch in the same general field as the starting switch as the present invention.
Among the several objects and features of this invention may be noted the provision of a motor starting switch, such as above-described, which eliminates the requirement for an internal jumper within the switch for providing energization of the starting or auxiliary winding of the motor;
the provision of such a switch which eliminates the requirement of timing the transfer of line current from the high to the low speed winding contacts of the switch (when the motor is selectively set in its low speed mode) upon movement of the switch from its starting to its run position and which effectively reduces arc energy disappated in one set of contacts within the switch thereby to increase the service life of the switch;
the provision of such a switch which limits the force exerted on the contacts within the switch when it is in its starting position thereby to prevent overstressing of the components in the switch and to prevent premature failure of the switch; and, in general,
the provision of a starting switch which has a greatly enhanced service life.